Contact lenses made from various polymeric materials have been investigated for a number of years. Such materials can generally be subdivided into two major classes: hydrogels and non-hydrogels. Non-hydrogels do not absorb appreciable amounts of water, whereas hydrogels can absorb and retain water in an equilibrium state. Hydrogels generally have water content greater than about five weight percent and more commonly between about 10 to about 80 weight percent. Regardless of their water content, both non-hydrogel and hydrogel contact lenses tend to have relatively hydrophobic, non-wettable surfaces. Contact lenses also readily absorb substances from the fluids that surround them. In an industrial setting this can have potentially serious complications as the contact lenses can absorb noxious chemicals and hold them in contact with the cornea for a substantial period of time. Unwanted absorption of substances may also result in discoloring of lenses or an uptake of preservatives, resulting in subsequent long-term hypersensitivity problems. The hydrophobic surface of the lens especially attracts binding of proteins via hydrophobic interactions. The protein layers adsorbed on the contact lens surface can blur vision and further get bacteria attach to them, causing infections, which can lead to permanent visual loss.
Surface structure and composition determine many of the physical properties and ultimate uses of solid materials. Characteristics such as wetting, adhesion and lubricity are largely influenced by surface characteristics. The alteration of surface characteristics is of special significance in biomedical applications, where biocompatibility and adhesion properties are of particular concern. Therefore, people have recognized the need for rendering the surface of contact lenses and other medical devices hydrophilic and resisting to biological binding. Increasing the hydrophilicity of the contact lens surface improves its wettability with tear fluid in the eye. This in turn improves the wear comfort of the contact lenses. A hydrophilic, neutral charge coating can also suppress protein and lipid binding to reduce eye infection. In the case of continuous wearing lenses, the surface is especially important. The surface of a continuous-wear lens must be designed not only for comfort, but also for avoiding adverse reactions such as corneal edema, inflammation, or lymphocyte infiltration.
Various patents have disclosed the attachment of hydrophilic polymer coatings to the contact lens surface in order to render the lens more biocompatible. For example, U.S. Pat. No. 5,652,014 (Galin et al., “Medicament coated refractive anterior chamber ocular implant”) disclosed amination of a contact lens substrate followed by reaction with polymers, such as a PEO star molecule or a sulfated polysaccharide. U.S. Pat. No. 5,344,701 (Gagnon et al., “Porous supports having azlactone-functional surfaces”) taught the attachment of oxazolinone or azlactone monomers to a substrate by means of plasma. The oxazolinone group is attached to a porous substrate by reaction of the ethylenic unsaturation in the oxazolinone monomer with radicals formed by plasma on the substrate surface. U.S. Pat. No. 6,200,626 (Grobe et al., “Surface-treatment of silicone medical devices comprising an intermediate carbon coating and graft polymerization”) disclosed coating silicone hydrogel contact lenses with a carbon-containing layer made from a diolefinic compound having 4 to 8 carbon atoms by the surface plasma treatment, and then graft polymerization of a mixture of monomers comprising hydrophilic monomers onto the carbon layer. U.S. Pat. No. 5,364,918 (Valint, Jr. et al., “Surface modification of polymer objects”) and U.S. Pat. No. 5,525,691 (Valint, Jr. et al., “Surface modification of polymeric objects”) disclosed a method of modifying the contact lens surface by adding a polymerizable surfactant to the monomer mixture. U.S. Pat. No. 5,352,714 (Lai et al., “Wettable silicone hydrogel compositions and methods for their manufacture”) disclose a method of improving silicone-containing hydrogels for enhanced wettability. U.S. Pat. No. 5,621,018 (Chabrecek et al, “Functionalized photoinitiators, macromers thereof, and the use thereof”) introduced functionalized photoinitiators onto contact lenses to modify surface by polymerization of photopolymerizable monomers. U.S. Pat. No. 6,440,571 (Valint, Jr. et al., “Surface treatment of silicone medical devices with reactive hydrophilic polymers”) disclosed surface treatment of silicon medical devices with reactive hydrophilic polymers. All the above methods for preparing coatings relates to free radical polymerization. However, free radical polymerization has limitations in the following aspects: (1) The polymerization happens both in solution and on the surface of the contact lens. The solution polymers could be physically attached to the surface of the contact lens, which eventually leaks into the eye, causing problems to the eye. (2) For free radical polymerization process, the majority of the polymer is formed in the solution. Both monomers and free polymers formed in the solution could be grafted to the surface of the contact lens, resulting in non-uniform polymer surface and no control on the thickness of the coating. (3) For free radical polymerization process, there is no order for the polymer thin film. In addition, there is no control of the polymer chain density.
It is desirable to have a method that is a much better control process for coating the surface of the contact lens than that of the free radical polymerization. It is also desirable to have this controlled process to generate an optically clear thin coating for the surface of a medical device that renders the device hydrophilic, biocompatible and suppressing binding of proteins and lipids onto the surface. Moreover, it is desirable to form a coating for contact lens that is capable of continuous wear for long time, preferably for a week or more and simultaneously tear-wettable and highly permeable to oxygen without adverse effects to the comea.